Since there is no "silver bullet" here, it doesn't matter much which product you use. You have the same challenges with any of them. I would recommend spending a few days on a Proof-of-Concept (PoC) re-write of 2-3 typical screens. Implement the POC for each "finalist" product, and see how it works out. Keep track of how long it took for each one, things that were easier/harder, and how the end result appears to the end-user (performance, good/funny-looking, robustness, "feel").
As for the actual re-write, I would recommend the following:
- Re-factor existing application to remove business logic from the UI.
- Full Regression testing, and push that into production.
- NOW proceed with conversion to one of the web tools.
Oops - I left out a step. Step 0: FREEZE all features/fixes. If fixes are needed to current production, they'll need to be done in a separate branch, and then rolled-up into this project later.
Note that this type of work lends itself nicely to outsourcing, as the work is straightforward and the requirements are simple. Especially if it can be delivered one form at a time, so progress, timelines, and $$$ can be measured in small chunks.
Another preliminary step is to develop a "cook book" for stripping the business logic from the existing GUI layer. It should identify naming conventions, common libraries (for code that should have been shared all along but wasn't), and should describe the conversion methodology.
